Molding Materials

Molding Materials

 

in founding, materials used primarily for the production of single-casting and semipermanent molds.

Molding materials are classified as component materials and mixtures, called molding sands; the former are used to make molding sands, core mixtures, and other compositions, such as foundry facings. Molding materials may also be classified as fillers, binders, and auxiliary materials. Fillers include quartz sands, highly refractory materials (such as chromite), and special-purpose materials (such as shot and shavings made from pig iron). Quartz sands are the most common fillers. The major constituent of these sands is quartz (SiO2), which is highly refractory (1710°C) and very hard (5.5–7 on Mohs’ scale).

In the USSR, sands are classified according to the percentage composition of the clay component (particles of clay and other minerals less than 22 microns in size) as quartz sands (class K; up to 2 percent clay) and clay sands (2–50 percent clay); the latter include class T (weak), class P (medium), class Zh (strong), and class OZh (very strong) sands. Molding materials containing more than 50 percent clay are considered clays. Quartz and clay sands are classified by grain size, determined by sifting the granular base through standard-size sieves. Very coarse and coarse sands are recommended for the production of steel and iron castings weighing more than 1,000 kg; medium sand is recommended for small and medium-size steel and iron castings; and fine and very fine sands are recommended for thin-walled castings of pig iron and steel, as well as for castings of nonferrous alloys. Fine sand is used in the production of individual piston rings. Powdered quartz is used in the production of foundry coatings for steel castings; zircon, chromite, magnesite, and other highly refractory molding materials are also used for this purpose.

The principal binding materials are clays. They may be classified according to mineralogical composition (for example, kao-linite and montmorillonite clays) or according to binding strength in the moist state (50–110 kilonewtons per m2, or 0.5–1.1 kilograms-force per cm2) or the dry state (200–550 kilonewtons per m2, or 2–5.5 kilograms-force per cm2); other classifications are based on thermal and chemical stability and plasticity. Clays that have kaolinite or montmorillonite as their major components are widely used as binders. Montmorillonite clays are components of highly adhesive clays known as bentonites. Other binding materials include synthetic resins (urea-formaldehyde resins, such as KF-60, and furan resins, such as FF-1S), bonding agents, and water glass.

Auxiliary materials that improve the quality of molding sands and castings include foundry facings, admixtures, materials for surface alloying, and other special materials. Foundry facings of powdered quartz or zircon, graphite, and talc prevent molding materials from adhering to castings. Admixtures for molding sands include hardeners (such as ferrochrome slag), foaming agents, and organic materials (such as sawdust). A hardener accelerates chemical hardening of the water glass molding and core sands; foaming agents make the sand fluid, which eliminates the need for packing. Organic admixtures increase the collapsibility of cores and molds and prevent the formation of cracks in the castings.

Materials for surface alloying impart special properties (such as high wear resistance) to the surface of the castings; they include tellurium, carbon, chromium, manganese, and silicon, which are added to foundry facings. Other special materials include molding powders, separating liquids, solvents, and fillers, such as sawdust and slag. Molding powders and separating liquids are used in molding to prevent molding sand from sticking to the pattern. Solvents make the synthetic resins fluid and thus ensure homogeneity in the molding sand. Sawdust and slag are poured into the middle of large mold cores to increase the cores’ collapsibility and gas permeability. Used molding sands may be treated by sifting and removing metal inclusions, and may then be reused for the production of new molding sands.

REFERENCE

Stepanov, Iu. A., and V. I. Semenov. Formovochnye materialy. Moscow, 1969.

M. N. SOSNENKO

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